304 Stainless Steel: The Workhorse Austenitic Grade

304 (UNS S30400, EN 1.4301) is the most widely used austenitic stainless steel, with roughly 18% chromium and 8% nickel. Its corrosion resistance comes from a thin, self-healing chromium-oxide passive film that reforms whenever oxygen is present. In the annealed condition 304 is essentially non-magnetic, although heavy cold working can induce a small amount of deformation-induced martensite and slight magnetism.

The grade offers excellent formability and weldability. The main metallurgical caveat is sensitization: holding the steel in the 450–850 °C range causes chromium carbides to precipitate at grain boundaries, depleting nearby chromium and reducing intergranular corrosion resistance. This is avoided by using low-carbon 304L, stabilised variants, or solution-annealing after welding. 304 cannot be hardened by quenching — its only heat treatment is annealing at 1010–1120 °C followed by rapid cooling.

Typical properties: tensile strength ≈ 515 MPa, yield ≈ 205 MPa, elongation ≈ 40%. It conforms to ASTM A240/A276, JIS SUS304 and EN 10088. Common uses include food and beverage equipment, architectural cladding, kitchenware, pharmaceutical tanks and heat exchangers.

For chloride-rich or marine service, 316L (with 2–3% molybdenum) is preferred because 304 has limited pitting resistance in the presence of chlorides. When sourcing 304, verify the mill certificate: some low-cost imports drift in nickel content, which directly degrades corrosion performance. Proper surface finish (2B, BA or pickled) also matters — rougher surfaces trap contaminants and accelerate localised corrosion.